Recently, with increasing awareness of environmental protection, the demand on renewable energy is growing. Among various renewable energy sources, solar energy is expected to replace fossil fuel as a new energy source because it provides clean energy without depletion. The solar energy may be converted into electric energy without generating contaminants. In other words, the solar energy source is the most viable renewable energy source.
Generally, a solar cell is used to convert solar energy into electric energy. The conventional solar cell is made of semiconducting materials. In particular, the silicon-based solar cell is the mainstream in the market. The photoelectric conversion efficiency and the cost-effectiveness of the conventional solar cell are gradually improved. However, the photoelectric conversion efficiency and the cost-effectiveness of the conventional solar cell are still unsatisfied so far. Consequently, many academic institutions and manufacturers devote much effort in improving the original solar cell configurations and looking for the novel solar cell configuration in order to increase the performance and reduce the fabricating cost. For example, a dye-sensitized solar cell (DSSC) is one of the candidate solar cells because the fabricating cost of the dye-sensitized solar cell is as low as one tens to one fifth of the fabricating cos of the conventional silicon-based solar cell. Moreover, since it is not necessary to produce the dye-sensitized solar cell under the high-temperature vacuum environment, the production conditions of the dye-sensitized solar cell are less stringent than the conventional silicon-based solar cell.
The dye-sensitized solar cell is a photoelectrochemical system that uses the photoexcitation of a dye-based photosensitizer to generate photocurrent through a chemical reaction. Generally, the dye-sensitized solar cell comprises a substrate, a transparent conductive film, a semiconductor film, a dye, an electrolyte and a counter electrode. The working principles will be illustrated as follows. Firstly, a titanium dioxide or zinc oxide semiconductor film is coated with a special light-absorbing dye. When the dye is irradiated by sunlight, the electrons of the dye are excited from the ground state to the excited state. The excited electrons may be injected into the conduction band of the semiconductor film so as to become free electrons. These free electrons may flow out through conducting glass or conducting plastic, which is connected with the semiconductor film. Consequently, a current is generated. The electrolyte is located at the other side of the dye. The dye molecules that lose the electrons may receive electrons from the electrolyte so as to be restored to its original state. Moreover, after the electrons are released from the dye, the electrons pass through a load and flow to a platinum electrode, which contacts with the electrolyte. Consequently, the lost electrons of the electrolyte are supplemented, and a complete loop is created. In the dye-sensitized solar cell, the structure of the photosensitive dye may influence the photoelectric conversion efficiency and the stability of the dye-sensitized solar cell. Moreover, the selection of the photosensitive dye is highly related to the cost of the dye-sensitized solar cell and even related to the development potential of the solar cell.
Therefore, the present invention provides a photosensitive organic dye for a photoelectric converting device such as a dye-sensitized solar cell. The balance between the photoelectric conversion efficiency, the stability of the solar cell, the ease of the production process and the cost effectiveness may increase the commercial competition of the dye-sensitized solar cell.